The present invention pertains to preamplifiers for magnetoresistive elements of mass data storage systems, particularly magnetic storage systems.
A typical high-performance magnetic storage system includes a magnetic disc for storing data, an inductive write element for writing data to the disc, and a magnetoresistive read element for reading, or retrieving, data from the disc. The write element writes binary data to the disc by magnetizing tiny regions of the disc to represent a sequence of ones and zeroes. The magnetoresistive (MR) read element has an electrical resistance that changes according to the magnetization of the tiny regions as they move past the MR element. These resistance changes are then translated initially to an electrical data signal and ultimately to the original binary data.
Translating the resistance changes to an appropriate electrical signal entails coupling the MR element to an electric circuit, known as a preamplifier. The preamplifier typically includes a current bias circuit that converts the resistance changes to a voltage signal and an amplifier circuit that amplifies, or scales, the voltage signal before outputting the resulting data signal to detection circuitry for further processing.
Specifically, the current bias circuit forces a constant bias current Ib through the MR element (or head), to establish a nominal (steady-state or DC) bias voltage, Vb, across the element. The bias voltage, Vb, which equals the product of the intrinsic head resistance Rh and the magnitude of bias current Ib (RhIb), serves as a constant electrical background for sensing the resistance changes. The resistance changes cause momentary positive or negative pulse-like variations in the head voltage. The amplitude of these pulse-like variations equals the product of the bias voltage (Vb or IbRh) and a constant factor k, or kIbRh. The constant factor k represents the percentage change in head resistance Rh caused by the magnetized regions.
The amplifier circuit of the preamplifier amplifies the pulse-like variations by a constant factor G, known as the gain of the preamplifier. The amplitude of the resulting voltage signal Vout equals the product of the gain G and the amplitude of the pulse-like variations, or GkIbRh. Thus, the typical preamplifier with constant-current biasing produces an output voltage signal Vout with an amplitude that depends not only on the gain G of the preamplifier, the percentage change in head resistance k, and the bias current Ib, but also on the head resistance Rh.
The dependence of the output amplitude on head resistance Rh is troublesome for two reasons. First, it is common, in multi-disc, multi-head storage systems, to use several MR elements with the same preamplifier. MR elements, which are quite small and thus difficult to manufacture, inevitably have unique resistances which vary significantly from each other, sometimes by a factor of three or more. Thus, because of its dependence on head resistance, switching from one MR element to another changes the amplitude of the output signal by a corresponding factor. Second, head resistance also varies with temperature, age, and wear. Thus, even in systems with only one MR element, the output amplitude still varies because of changing head resistance. Amplitude variations are highly undesirable because they prevent detection circuitry from performing consistently with the signals from each MR element.
Additionally, the dependence of the output amplitude on the preamplifier gain G can also be troublesome. Although the preamplifier gain G should ideally be constant, it may vary with temperature changes in the preamplifier. Gain variance ultimately stems from the effect of temperature on transistors in the amplifier circuit. This gain variance is undesirable because it changes the amplitude of the output signal, as the preamplifier warms and cools during its operation. Thus, gain variance also undermines consistent performance of the detection circuitry.
Therefore, to promote consistent performance of magnetic storage systems using magnetoresistive read heads, there is a need for a constant-current preamplifier having an output signal amplitude not only independent of head resistance, but also independent of temperature.